1. Field of the Invention
This invention relates to heat transfer ink sheets used for heat-sensitive transfer recording.
2. Description of the Prior Art
Owing to the development of thermal heads, heat-sensitive recording methods have been widely adopted in facsimile apparatus and printers. In these methods, there is used a heat-sensitive recording paper which has generally a heat-sensitive color-forming or chromogenic layer in which two ingredients capable of developing a color on heating are dispersed, the layer being formed on a paper. However, this type of recording paper has the drawbacks that it is unsatisfactory in preservability, it is liable to deteriorate after recording, and it has a poor resistance to solvents. There has been proposed a heat-sensitive transfer material (a heat transfer recording sheet) which has overcome the above drawbacks. The transfer material so proposed has a heat-fusible or hot melt ink layer formed on a support. For recording, the transfer material is superimposed on ordinary paper or the like and subjected to heat generated from a thermal head so as to transfer the ink to the ordinary paper or the like. According to the above recording method, recording on ordinary paper is possible and thus, the drawbacks involved in the known heat-sensitive recording methods can be overcome.
In this recording system, heat generated from the thermal head permits the heat transfer ink to be melted through a support so as to transfer the melted ink to the ordinary paper. Heat transfer ink sheets have been heretofore made by several methods including a hot melt coating method in which a hot melt ink mainly composed of a colorant such as a pigment or dye, a wax and a resin is applied onto a base film and a hot lacquer coating or solvent coating method in which an ink dispersed in a solvent is heated and applied onto a base film. Although the hot melt coating is effected by applying an ink which is a solid at normal temperatures but is turned into liquid on heating while melting the ink by heating, it is disadvantageous in that for example, it will produce an irregular coating on a surface to be coated and that it needs an additional specific apparatus for making a transfer sheet on which different kinds of color inks are selectively applied. The hot lacquer or solvent coating is effected by applying an ink diluted with solvents while heating to a temperature not higher than the melting point of the ink. Japanese Laid-open Patent Application No. 58-128897 discloses a method which comprises applying at normal temperatures an ink having 10 wt% or more of wax dissolved in solvents at normal temperatures. Japanese Laid-open Patent Application No. 59-57791 discloses an improved method in which inks ixed with solvents are subjected to evaporation of the solvents at temperatures below the melting points of the inks and then heated and melted for mixing. These coating methods using solvents as diluents have an advantage in that they allow conventional existing gravure or flexo printers to be utilized. However, several disadvantages are involved. More particularly, since an ink is applied in the form of a dispersion in a solvent in the above methods, irregularities of the resulting coating on the substrate surface can be lessened as compared with the hot melt coating method, but wax is dispersed along with a colorant such as a pigment while being partially dissolved. As a result, even though the wax is very finely dispersed and coated, the irregularities of the coating on the surface cannot be lessened to a satisfactory extent with poor dispersion stability of the wax and pigment. This will lead to the poor stability of the ink at the time of its application by printers and also to settling or sedimentation of the wax and pigment during their storage, thus resulting in poor preservability.
In addition, Japanese Laid-open Patent Application No. 59-114098 describes a heat-sensitive transfer sheet which comprises a base film and a hot melt ink layer and a hot melt wax layer, these layers being formed on the base film in this order. Probably since this heat-sensitive transfer sheet has the ink layer composed mainly of wax and the wax layer superimposed thereon, it will provide a print or record which is not very good because of in sharpness.